1. Field
The present embodiments relate to an imaging device including a noise reduction process function removing noise generated when an exposure is performed for a long time.
2. Description of the Related Art
In recent years, an electronic camera having a noise reduction (NR) mode is known as the electronic camera including an imaging sensor (for example, refer to Japanese Unexamined Patent Application Publication No. 2005-79948). Generally, the electronic camera performs a dark imaging with an exposure time which is the same as an exposure time of a normal image under a state in which the imaging sensor is light shielded just after the normal image is photographed, in this noise reduction (NR) mode. Subsequently, a correction process of data of the normal image is performed by using data of the dark image inside the electronic camera. Image degradation such as a dark current noise generated at the imaging sensor and a minute scratch (for example, a point scratch) caused by a pixel defect unique to the imaging sensor is reduced by this correction process.
However, there are problems as stated below in the noise reduction (NR) mode. The dark current is generated by heat, and therefore, dark current amounts are different if temperature of the imaging sensor changes even if the exposure times are the same (the dark current amount becomes double caused by the temperature rise of 7° C. to 10° C.). Accordingly, an error occurs between respective dark current components if the temperature difference exists between a normal exposure time and a dark exposure time. There is a tendency that fluctuation of temperature becomes large and a dark current difference also becomes large as the exposure time becomes long.
Generally, when the imaging sensor is used continuously from the normal exposure to the dark exposure, the temperature rise of the imaging sensor is further incurred at the dark exposure time. The dark current also increases according to the temperature rise. As a result, a problem occurs in which the dark current is too much subtracted in the correction process to thereby incur a black saturation (excessive correction of dark level) and so on.
Besides, when a long time exposure is performed after a live view mode displaying a live video of the imaging sensor on a liquid crystal display is used for a long time, the long time exposure is started under a state in which the temperature rises up to a saturation temperature caused by the live view operation. In this case, power consumption is small and the temperature decreases rapidly because a mode shifts to a stand-by mode during the long time exposure. Accordingly, the dark current becomes small in the dark exposure than the normal exposure as it is opposite to the normal state. In this case, a problem occurs in which a black floating (shortage of correction of dark level) is incurred caused by a shortage of the correction amount in the correction process.
Further, it is known that the dark current noise such as a white point occurs when the exposure is performed for a long time. In this case, a problem occurs in which the white point remains if the dark current amounts are different between the normal exposure time and the dark exposure time.
Here, in the noise reduction (NR) mode disclosed in the Japanese Unexamined Patent Application Publication No. 2005-79948, it is estimated that the temperature rise occurs when an exposure time determined in advance has passed. However, there is a room for further improvement in a point that the correction cannot be performed enough when the predicted temperature rise does not occur actually.